Recently their has also been a break through to allow siri from the iPhone 4s to be grafted onto the iPhone 4 too.

Their are instructions on how to do this via google and at the moment their is a company offering access to their siri proxy for $20 / £13 per year.

A word of warning..........

before you attempt to carry out jail breaking and adding siri please do your homework regarding the ups and pitfalls on carrying out this procedure.
And as important , how to rectify issues that can occur after doing this.

In order to jailbreak your iPhone 4s use a genuine Unlock iPhone software. This is one of the most easiest and convenient way to get your iPhone unlocked.

Jailbreaking is different from unlocking. Jailbreaking just removes the limitations imposed by Apple on iOS. Unlocking is a process of removing network restriction enforced by network providers. So jailbreaking never removes network restrictions.
The safest and easiest way to unlock an iPhone is to unlock through remote unlocking method. It is also an affordable method too. The remote unlocking for an iPhone can be acquired from Online GSM Unlock.com at affordable cost with easy unlocking instructions.